The present invention relates in general to manifold multiplexers and pertains, more particularly, to a multiplexer comprising a number of filters connected to a manifold and designed so that there is no need for extra immittance compensation networks.
There are different approaches to the design of multiplexers, particularly those coupling to a manifold. Due to requirements in communication satellites and other applications, attempts have been made to produce manifold multiplexers. One important and difficult requirement is to provide an output multiplexer on a waveguide manifold with bandpass channels separated to yield guard bands of only say, for example, 10%. Most design techniques have adopted an approach based upon singly terminated bandpass channels resulting in 3 dB crossover points between channels. See for example the articles of R. J. Wenzel and W. G. Erlinger, "Narrowband contiguous multiplexing filters with arbitrary amplitude and delay response", 1976 IEEE MTT-S International Microwave Symposium Digest, IEEE Cat. No. 76CH1087-6MTT, pp. 116-118; and M. H. Chen, F. Assal and C. Mahle, "A contiguous band multiplexer", Comsat Technical Review, Vol. 6, No. 2, pp. 285-305, Fall 1976. These designs exhibit good return loss over the channel bandwidths and the guard bands. Dummy channels typically have to be included to simulate channels at the edges of the total multiplexer bandwidth, forming additional annulling networks. Thus, in the prior art it is quite common to require redundant elements to provide channel interaction compensation to produce a channel performance comparable to the individual channels based upon a singly terminated prototype.
The need for contiguous band multiplexers originally arose in receiver design for countermeasures where the incoming signal was unknown and complete band coverage was necessary with good match at all frequencies. In this case all channels have to be designed on a singly terminated basis and must provide a prescribed level of attenuation over the major part of other bands.
However, the requirements for multiplexers in communication systems are different since they must provide good reflection and transmission only over each channel bandwidth whilst maintaining high attenuation over all other channels. For typically realizable passband return loss specifications (e.g. 20 to 25 dB) the optimum solution leads to attenuation in excess of 10 dB at the crossover frequencies independent of the guardband bandwidth. Thus, contiguous band multiplexers are non-optimum in this situation and have probably been used because a design procedure was known. They result in a higher degree filter than necessary in each channel in addition to the annulling network. Furthermore, if one attempts to use singly terminated designs for crossover levels in excess of 3 dB, the passband return loss rapidly deteriorates if further annulling (immittance compensation) networks are not used.
The significant difference between the optimum design approach and the continguously designed solution to the above type of problem may be illustrated by the simpler diplexer examples presented in J. D. Rhodes, "Direct Design of Symmetrical Interacting Bandpass Channel Diplexers", I.E.E. Journal on Microwaves, Optics and Acoustics, Vol. 1, No. 1, pp. 34-40, September 1976. See in particular FIGS. 4 and 5 of that article. For the case of two non-contiguous bandpass channels of bandwidth 2 and guardband of 1, 5-cavity Chebyshev filters may produce a return loss 25 dB over each passband whilst 34 dB attenuation is achieved over the opposite channel, whereas the contiguous approach using similar 5-cavity filters gives an attentuation level over the opposite channel of only 19 dB. Thus the price paid for preserving the 25 dB return loss at the common port over the guardband region is a 15 dB reduction in the attenuation level in the stopband. Hence, for most applications in communication systems, contiguous band multiplexers result in designs far from the optimum solution.
Accordingly, it is an object of the present invention to provide an improved manifold multiplexer and one in particular that can be used for contiguous band operation. In accordance with the present invention the manifold multiplexer forms phase shifters displacing each of the filters forming the multiplexer with these phase shifter networks also forming immittance compensation means.